Timelike Infinity

Timelike Infinity: the strange region at the end of time where the Xeelee, owners of the universe, are waiting... The second novel in Stephen Baxter's Xeelee sequence. First there were good times: humankind reached glorious heights, even immortality. Then there were bad times: Earth was occupied by the faceless, brutal Qax. Immortality drugs were confiscated, the human spirit crushed. Earth became a vast factory for alient foodstuffs.
Into this new dark age appears the end of a tunnel through time. Made from exotic matter, it is humanity's greatest engineering project in the pre-Qax era, where the other end of the tunnel remains anchored near Jupiter. When a small group of humans in a makeshift craft outwit the Qax to escape to the past through the tunnel, it is not to warn the people of Earth against the Qax, who are sure to follow them.
For these men and women from the future are themselves dangerous fanatics in pursuit of their own bizarre quantum grail. Michael Poole, architect of the tunnel, must boldly confront the consequences of his genius.